Vascular smooth muscle (VSM) membranes from rat aorta are densely supplied with a 225 pS, Ca2+-activated K+ channel (IK(Ca) channel), which resembles the "maxi-K" or "Big K" channel. In patch-clamped aortic VSM from spontaneously hypertensive rats (SHR), these IK(Ca) channels show an increased open state probability (NPO), an enhanced Ca2+-sensitivity, and a higher current density compared to aortic VSM from control Wistar Kyoto rats (WKY). this provides initial evidence that IK(Ca) is increased in VSM in hypertension. However, alterations in SHR VSM may reflect genetic changes unrelated to the hypertension rather than alterations related directly to the elevated pressure. To clarify this, we have obtained thoracic aortic VSM from Sprague Dawley (SD) rats exposed to high pressure after inter-renal aortic coarctation (IR-AC) or reduction of renal mass (RRM). Aortic VSM from IR-AC showed an elevated membrane density of IK(Ca) and profound contraction after pharmacological block of IK(Ca), implicating enhanced IK(Ca) as a critical regulator of VSM reactivity in IR-AC VSM. However, RRM aortic VSM showed no evidence of increased IK(Ca) despite exposure to elevated blood pressure. Since the IR-AC has high circulating levels of angiotensin II (AII) and aldosterone (Aldo), while the RRM has reduced levels of both AII and Aldo, one possibility is that > normal AII and/or Aldo levels are required for pressure-induced increases in VSM IK(Ca). This hypothesis will be tested in this Project by surgically coarcting aortas of SD rats at different sites to provide six aortic VSM preparations exposed to different chronic levels of in situ blood pressure and AII/Aldo. To determine the mechanisms for enhanced VSM IK(Ca) in hypertension, IK(Ca) single-channel conductance, NPO, Ca2+- sensitivity, and membrane density, as well as measurements of cytosolic free calcium levels, will be compared between different aortic VSM preparations. These results will be compared with the acute effects of AII and Aldo on IK(Ca) in aortic VSM patches. Further, the potential of enhanced IK(Ca) to limit VSM excitability will be investigated in aortic segments, and in in vitro and in situ small pressurized arteries from the same coarctation models. This integrated approach of defining VSM IK(Ca) mechanisms, from the level of the single-channel to the in situ blood vessel, will provide new and critical information on the mechanisms and physiological role of this channel in hypertension.